1. Field of the Invention
The present invention is in the field of mechanical testing and specifically relates to apparatus for applying a known axial force to a valve stem or to a shaft, the ends of which are not accessible.
At numerous points in a nuclear power plant, the flow of a fluid is controlled by large gate valves. When a considerable pressure difference exists between opposite sides of the gate, frictional forces become large and a large force must be exerted by the valve stem to move the gate. Under emergency conditions the pressure difference may reach very large values, and there is concern as to whether the valve will operate or can be operated safely under those conditions. In particular, there is concern about the force on the valve stem or other components.
FIG. 1 shows the type of valve on which the present invention is used. The operator (actuator) is connected to the valve by a yoke, and the valve stem is located between the two legs of the yoke. Obviously, the force in the valve stem is equal to the sum of the force in the two legs, but opposite in direction. It was well-known, even before U.S. Pat. No. 4,805,451, to install a strain sensor on one of the legs of the yoke to measure its elongation, from which the stress in the valve stem can be calculated based on knowledge of the size and material of the legs of the yoke.
With so much at stake, it would be highly desirable to provide for an independent check of the calculated force in the valve stem. This can be done by calibrating the strain sensor(s) mounted on the leg(s) of the yoke, or other sensors mounted at appropriate points on the valve. The calibration consists in applying a known force to the valve stem and noting the corresponding reading(s) of the strain sensor(s).
Once the reading of the strain sensor has been found for known amounts of force applied to the valve stem, there is a basis for believing that a particular reading of the strain sensor implies a particular amount of force on the valve stem.
2. The Prior Art
A search of the prior art revealed that little has been done in solving the narrow problem of applying a known calibrating force to a valve stem. Accordingly, the search was broadened to include analogous problems in other fields, with the following results.
In U.S. Pat. No. 4,805,451, Leon shows that the thrust on a valve stem can be derived from the readings of strain sensors mounted on the yoke of a valve. Leon states that the yoke strain sensor may be calibrated by applying a known force to the valve stem of a fully-assembled valve, but he doesn't suggest how this can be done. The Leon patent can be understood as indicating the desirability of the present invention.
In U.S. Pat. No. 4,911,004, the same Leon teaches the use of Poisson's ratio to relate the elongation or compression of the valve stem to a reduction or increase in its diameter. The valve stem is grasped by a large C-clamp that is instrumented for detecting changes in the diameter. As can be appreciated, the accuracy of this approach is limited because of the minute changes in diameter that need to be measured.
Probably the closest prior art is U.K. Patent Application No. 2,086,063A by inventor Boast. He shows an hydraulically powered (26) set of jaws 19 that clamps onto the mid-section of a vertically extending I-beam. Hydraulic rams (3, 4) exert a force along the I-beam relative to the ground. The force is derived from the hydraulic pressure at the time the I-beam is lifted off the ground.
In U.S. Pat. No. 3,965,736, Welton, et al. describes devices for measuring strain in the polish rod of a well pumping unit, but does not include means for applying a calibration thrust. In a first embodiment two blocks are connected by a tube that extends parallel to the polish rod. The blocks are attached to the polish rod by set screws. A strain gauge is attached to the tube to measure its elongation. In a second embodiment, hinged jaws are connected by an axially-extending tubular sleeve to which a strain gauge is attached. In a third embodiment, a pipe clamp with a strain gauge attached to it is used. None of these three embodiments resembles the present invention, and Welton, et al. do not include means for applying a calibration thrust.
In U.S. Pat. No. 4,607,534, Cerbone describes apparatus for determining the force with which a pin pushes against a surface. He uses a hydraulic ram to push the pin away from the surface and an electrical circuit to determine when contact between the pin and the surface is broken.
In U.S. Pat. No. 3,942,368, Hoyt shows an apparatus for testing the holding power of earth anchors by pulling upward on them. The force is derived from measurement of the hydraulic pressure. The end of the hydraulic ram is provided with a clevis and the end of the anchor bolt is provided with an eye. The clevis pin passes through the eye to fasten the ram to the anchor bolt. Unlike the present invention, the invention of Hoyt requires access to the end of the bolt or shaft and thus is not directly applicable to the present invention.
In U.S. Pat. No. 3,954,005, Edwards shows a part having an inwardly-facing frusto-conical member that surrounds a length of rebar being tested. Two or more partly annular gripping wedges fit between the frusto-conical member and the rebar.
In U.S. Pat. No. 4,594,900, Pellerin, et al. use a hydraulic vice for gripping the ends of a length of lumber for applying longitudinal compressive and tensile loads to it.